I know that some people may be able to provide their own source of fuel, but for those buying the biomass, is it a cheaper system to run than conventional boilers?

I think it's more expensive, actually, unless you put something like coal in it, which would defeat a lot of the purpose. Coal is great for storing energy, which is why it's widely used as a fossil fuel. Other than that, things like wood are very expensive when you consider how quickly they burn out and the small return of energy.

They're only a good investment if you create a lot of waste that's burnable.

I have a friend who powers her biomass boiler using purely her rubbish. Her needs aren't very high; she lives alone and doesn't spend much time in the house. She does generate a lot of waste though, so this has been a Godsend for her. I'm not sure I'd be able to manage the same, but I bet it feels satisfying burning your rubbish and saving on your energy bills!

Something people don't realise as well is that burning things like paper can be completely carbon neutral. If the trees that are cut down are from sustainable areas, the trees that grow to replace them will soak up any CO2 released by burning their predecessors. It's a cycle.